This disclosure relates generally to fuel cells and methods of their manufacture and use. More particularly, the present disclosure relates to fuel cells capable of operation by electrolyzing compounds in a biological system and methods of their manufacture and use.
There has long been interest in techniques for providing electrical power from a power source or for producing hydrogen from a liquid or solid source that utilizes biological matter freely available in the environment. One such area of interest is in developing microbial fuel cells (MFCs) as a means to treat wastewater more efficiently by breaking down organic waste products and converting the energy of their chemical bonds into electricity and hydrogen. According to the May 2004 issue of Environmental Science & Technology, 46 trillion liters of household wastewater are treated annually in the United States at a cost of $25 billion. Importantly, the electricity required—mostly for aeration—constitutes 1.5 to 2% of the electricity used in the nation. Other nations have similar statistics.
Recently, researchers have shown the feasibility of using microbial fuel cells to generate electricity wherein the source of electricity is the chemical energy contained in the bonds of organic compounds which are a principle constituent of wastewater. Using laboratory scale microbial fuel cell reactors comprising a special anode, a simple cathode and a suitable proton exchange membrane (PEM) to separate the wet anode and cathode portions of the microbial fuel cell, energy densities in the order of 30 watts/cubic meter have been generated. Expectations are that power densities will increase as the efficiency of the cells can be improved by advances such as those provided by the present fuel cell.
The process uses bacteria, living in biofilms on the special anode, to break down the organics contained in a biostream, separating electrons from protons. These electrons and protons then travel to the cathode, the former via an external wire, the latter by diffusing through the electrolyte which is generally a substance that permits effective movement of small molecules and atoms while at the same time the electrolyte does not effectively conduct electricity. In the electricity-generating microbial fuel cells, the protons and electrons combine at the cathode with oxygen to form water. This consumption of the electrons allows more electrons to keep flowing from the anode to the cathode as long as there is a source of chemical bonds (i.e. organic waste) to fuel the reaction.
The early microbial fuel cells produced between 1 and 40 milliwatts of power per square meter (mW/m2) of anode electrode surface area. In the past few years researchers have been able to increase this more than 10 fold by demonstrating that they could generate power in the range of up to 500 mW/m2 using domestic wastewater and 1,500 mW/M2 with a surrogate for waste water comprising glucose and air. However, improvements to the output power density by another factor of at least 10 will be required in order to make the technology attractive on a commercial scale.